Hitherto, a NOx sensor has been known as a thick-film multilayer gas sensor of the limiting current type. The NOx sensor includes a sensor device (element) for detecting the concentration of NOx in a gas to be measured (i.e., a gas under measurement), and a heater disposed adjacent to the sensor device. The sensor device includes an inner pump electrode and an outer pump electrode disposed on the inner side and the outer side of a base member, respectively, which is made of a solid electrolyte layer having oxygen-ion conductivity. The inner pump electrode introduces the gas under the measurement into a first chamber that is exposed to the outside, and then pumps out oxygen in the gas under the measurement from the inner pump electrode to the outer pump electrode. The gas under the measurement, from which the oxygen has been pumped out, is introduced to a second chamber where NOx in the gas under the measurement is reduced to generate oxygen. The generated oxygen is detected to detect the concentration of NOx. The heater serves to perform temperature adjustment by heating the sensor device and holding it at a predetermined temperature.
In one proposed example of the above-described NOx sensor, a porous protective layer made of alumina, for example, is disposed so as to cover the outer pump electrode (see Patent Document 1). According to the proposed NOx sensor, the porous protective layer serves to prevent an oil component, etc., which are contained in the gas under the measurement, from being attached to the outer pump electrode.
In another proposed example of the above-described NOx sensor, a porous alumina sintered compact having communicating pores with a mean pore size of 500 to 1100 Å and a porosity of 6 to 16% is formed as an electrode protective layer on the surface of at least one of a pair of electrodes of a pump cell (see Patent Document 2). According to the proposed NOx sensor, even after use for a long period, it is possible to prevent a reduction of an output in detection of the gas under the measurement, and to ensure a stable detection output of the gas under the measurement. Similar techniques are further described in Patent Documents 3 and 4.    Patent Document 1: JP2008-164411A    Patent Document 2: JP 2005-156471A    Patent Document 3: JP2005-195516A    Patent Document 4: JP2006-343297A